α7 nAChR and Gprin 1 Interaction Effects on Neuroregeneration

Neuroregeneration

What would happen to our bodies if cells that are damaged never repair themselves?   Unfortunately, our body structures would eventually break down and no longer function correctly.  Luckily, with vast research conducted, there is a solution!  Neuroregeneration, the regrowth or repair of neural tissues, is imperative to maintaining functions within a system.  By repairing or replacing damaged cells, the damaged area has hopes of gaining back normal functioning.  The regeneration of neural cells has proven to be critical in helping patients with spinal cord injury.   Researchers hope to also find cures to neurodegenerative diseases such as Alzheimer’s and Parkinson’s using neuroregeneration.

Jacob Nordman is a Ph.D. candidate working in the Kabbani Lab of the Krasnow Institute at George Mason University.  He has done extensive research on α7 nAChR and Gprin1 interactions and how they regulate axon and growth cone development, navigation, and regeneration.  He presented his research on α7 nAChR and Gprin1 interactions within hippocampal neurons.

α7 nAChR Along with Gprin 1 Contribute to Axonal Growth

Growth cones, which contain F-actin and microtubules, are situated on axons and dendrites.  They are crucial in guiding axonal development, which allows for the precise wiring of neural circuitry.  To help you picture what a growth cone is, imagine it as an arm and a hand.  There are three layers to a growth cone: central zone (forearm), transitional zone (palm), and peripheral zone (arm used to pull a growth cone to its destination).  Growth cones work with gradients, where a guidance cue can result in either growth or retraction.  The process during which growth cones guide axonal development has seven states:  initiation, formation, guidance, branching, turning, arrest, and retraction.    

The α7 nAChR receptor is a ligand gated calcium channel which has a high affinity for nicotine and ACh, and has been linked to illnesses such as schizophrenia and Alzheimer’s due to its high expression in the hippocampus.  Although there have been studies emphasizing the contribution of α7 nAChR to neuroregeneration, the mechanisms behind this process have not been fully understood.  Gprin1 (G protein regulated inducer of neurite outgrowth 1) is a membrane-bound protein that is highly enriched in growth cones.  Nordman’s research focuses on how α7 nAChR along with Gprin 1 contribute to axonal growth and navigation in hippocampal development.   

Using fluorescent markers to detect proteins and their antibodies within hippocampal brain slices indicated that α7 nAChR and Gprin 1 had the highest expressions during early development at the soma and growth cones, where the cytoskeletal growth demands are highest.  During the early stages of development, neurons are still migrating to their final destinations and are highly dependent on growth cones to guide the paths.  To confirm strong and high overlap of α7 nAChR and Gprin 1 in the soma and growth cones, transfection was used to insert α7 nAChR into Neuro-2a cells, which are neuron-like cells that produce only Gprin 1.  Afterwards, to observe the interactions between the two proteins, immunoprecipitation was used to remove Gprin 1 from the Neuro-2a cells, in order to isolate α7 nAChR.  Upon removing Gprin 1 with siRNA, the direct interaction between α7 nAChR and Gprin 1 weakened, confirming not just the presence of both α7 nAChR and Gprin 1 in the soma and growth cone, but also that there is an active and direct interaction between the two.  

Immunoprecipitation Process

Another experiment on the relationship between α7 nAChR and Gprin 1 was done and it was found the Gprin 1 is a master switch for α7 nAChR.  When there was more Gprin 1, there was an increased growth in axons, more branching, as well as a higer surface area of the growth cones.  Conversely, when there was less Gprin 1, the growth cones retracted and shrinkage was seen.  These results pointed out that whatever is done to Gprin 1 offsets what happens to α7 nAChR.    

The research done by Nordman suggests that α7 nAChR and Gprin 1 together play a critical role in axonal development in the hippocampus.  This study furthers the field of neuroregeneration and can be used to develop drugs to repair neural damage by having more effective ways to regenerate axons and quickly guide them to their destinations.   The overactivation of α7 receptors has been cited in Alzheimer’s disease, causing axons to retract and neural cells to die.  Targeting α7 receptors with antagonists would surely result in useful treatments for Alzheimer’s or other neurodegenerative illnesses.  Myelination is the formation of a layer called the myelin sheath around the axons of neurons.  Myelin increases the speed of impulses through the axon and stabilizes it, which inhibits the regeneration of axons and branching of growth cones.  Thus, inhibiting myelin promotes movement within growth cones.   Inhibiting myelination increases regeneration of axons which is needed to treat neurodegenerative illnesses.  However, can this sort of treatment be a double-edged sword?  This may cause damage to myelin, which contributes to multiple sclerosis.  Neurotrophins are a class of growth factors which contribute to the survival and development of neurons.  Possibly with more studies done on neurotrophins and their interactions with myelin, more safe and effective treatments can be created to treat neurodegenerative illnesses.  

 Time-Lapse Imaging of a Growth Cone

The Role of Translational Research in Medicine

From Bench to Bedside

What is the point of research if it is reserved to only laboratories?  It is when research is practically applied to treat illnesses and increase society's knowledge of human health that research has achieved one of its ultimate goals.  Translational research aims to do just this, to create meaningful health outcomes from bench to bedside.  Scientific discoveries begin at the "bench" where scientists study diseases at the molecular level.  This research then progresses to clinical applications at the patient's "bedside" to improve their well-being.  Dr. Robert Lipsky, Director of Translational Research in the Department of Neurosciences at Inova Fairfax Hospital, founded the Inova-GMU Neuroscience Translational Research (INTR) Laboratory.  Dr. Lipsky's research revolves around functional genomics and its role in translational medical science. 

Genes play an important role in determining drug effects

Genetics makes each and every one of us unique, whether it is in our eye color or our susceptibility to illnesses. Medication too, should be tailored to each individual's needs.  Pharmacogenetics is the study of how genetic variations affect patients' individual responses to drugs, both therapeutic and adverse effects.  This study allows for the design of drug therapy based on patients' individual genetic profiles, as well as the analysis of the safety and efficacy of drugs.  

Dr. Lipsky regarded to Warfarin as an example of the role of pharmacogenetics in translational research.  Warfarin is a drug used to inhibit the formation of blood clots, however it has a complex dose-response relationship.  Research has found that variations in the vitamin K epoxide reductase complex 1 (VKORC1) and Cytochrome P450 CYP2C9 genes both affect patients' responses to Warfarin.  Knowledge of the role of genotype variation in response to Warfarin was a biomarker for determining the ideal dosage for individual patients. In another study for the development of biomarkers in Alzheimer's disease, GRIN2B SNP along with neuroimaging were established as biomarkers for mild cognitive impairment and neurodegeneration.  

To better treat Major Depressive Disorder (MDD) patients, the Sequenced Treatment Alternatives to Relieve Depression (STAR*D) Project was developed to find more effective treatments for patients who did not improve after an initial treatment with Citalopram, an antidepressant of the selective serotonin reuptake inhibitor (SSRI) class.  In the STAR*D Project, patients' DNA were screened for long and short forms of HTTLPR, a serotonin transporter. It was found that S and Long G (LG) alleles express low levels of serotonin transporters.  Patients with S and/or LG alleles had earlier and greater side effects with the SSRIs and were considered "poor-responders" to the treatments due to the adverse side effects.  

Spinal cord injuries occur more than some of us may think.  Here are some surprising statistics about spinal cord injuries:  about 250,000 Americans have spinal cord injuries (SCI) and about 52% of spinal cord injured individuals are considered paraplegic and 47% quadriplegic.  Although this is a growing problem, there has been little progress made to help patients with SCI.  Treatments that include steroids or surgery failed to improve SCI symptoms.  Fortunately a device called the oscillating field stimulator (OFS) was developed to promote the regeneration of the spinal cord and neurological recovery.  Along with stimulating axonal growth, the OFS device improves sensation in patients with complete SCI, the worst-case scenario for the injury, and overlays the injury by creating an "electric feel."  15 weeks after the implant of the device, patients experienced fewer urinary tract infections, fewer pressure sores, improved bladder and bowel control, and had better physical therapy and rehabilitation outcomes. 

Pets are also benefiting from new spinal cord injury treatments

 Understanding the relationship between genes and symptoms is the core of pharmacogenetics, helping enhance the field of translational research in developing drug therapy. Translational research has benefited the medical field greatly.  Scientists provide clinicians with effective methods to treat patients.  The overall effect of the treatment on the illness is observed, and may spark leads for new studies.  It is an ongoing cycle of discoveries and application, science being used for the greater good of humanity.  In order to enhance translational research, scientists must take initiative if they find that a treatment or device shows medical potential by developing clinical trials to test the efficacy of the treatment or device.  Translational science is the bridge connecting scientific research to clinical practice.  The road to achieving this goal is not easy however.  How can translational research be improved to strengthen and accelerate the process from bench to bedside?  Solutions that should be utilized include:  finding a way to improve and advance multidisciplinary research teams and creating new research tools to encourage the development of new ideas.  As translational research continues to advance, so will the health of humanity.

 

The Power of Translational Research

The Underlying Mechanisms of Neurological Diseases

 Dopamine controls the areas of the brain responsible for desire, decision-making, and motivation.

Have you ever tried to break a bad habit, such as drug addiction, but was not successful?  Or have you been rather successful in developing good habits, such as perseverance or eating healthily?  Whichever case it is, thank dopamine!   

Dr. Kim 'Avrama' Blackwell from the Computational and Experimental Neuroplasticity Laboratory of the Krasnow Institute at GMU has done research on the computational modelling of neurological diseases.  Of high importance in the development of these diseases is dopamine, a neurotransmitter involved in habits, addiction, and reward-centered behaviors.  Of the various illnesses affected by dopamine, common ones include: ADHD, schizophrenia, and Parkinson's. 

Dopamine is produced in various structures of the brain, including the substantia nigra (SN).  In the basal ganglia circuitry, the SN sends dopaminergic pathways to the striatum, which is then directed to the ventrolateral thalamus and primary motor cortex.  Since dopamine promotes movement, if neurons within the SN die and the SN diminish in size, movement is inhibited.  This is clearly depicted in patients with Parkinson's disease who have slow movement and rigidity of the body.  Dopamine is not only constricted to movement processes, but is also involved in the formation of habits, skill learning, and reward learning. 

Dopamine and basal ganglia circuitry differences in healthy individuals vs. those with Parkinson's.

As mentioned above, dopamine is responsible for learning and it does so by inducing synaptic plasticity.  A question posed by Dr. Blackwell was how do neurons know to increase certain synapses and not others?  The main component of synaptic plasticity is calcium, however it cannot do the job alone.  In order to understand the signaling pathways involved in synaptic plasticity, temporal and spatial specificity must be observed.  Long-term depression (LTD) and long-term potentiation (LTP) have been found to be underlying factors in cognitive functions such as learning and memory by way of synaptic plasticity.  In terms of spatial aspects, protein kinase A (PKA) location is important for LTP within the striatum.  PKA must be anchored near the enzyme adenyl cyclase help produce spatial specificity.  It is typically known that high frequency stimulation (~100Hz) produce LTD.  In the study conducted by Dr. Blackwell, it was found that theta burst stimulation produced striatal LTP.  This temporal pattern differentiated for LTD or LTP in the striatum.  In order to further discriminate temporal patterns, other molecules and pathways were observed.  Dopamine interaction with ACh activated Gq pathways, which resulted in the endocannabinoid 2AG in LTD and protein kinase C in LTP.  LTD and LTP are expressed in synaptic models and are shown to affect the amount of plasticity induced., thus impacting learning and memory. 

The mechanisms behind LTD and LTP are still not fully understood.  Their activities are dependent upon which circuits they operate and temporal, as well as spatial, specificity.  There can never be an end to enhancing computational modeling of neural networks to further understand the interactions of different molecules in various pathways.  In order to better the research that has been produced, Dr. Blackwell plans to evaluate responses of cortical activation with various levels of dopamine and derive plasticity rules to incorporate into models of striatal networks.

During Dr. Blackwell's lecture, I can surely affirm that LTP was at work in order for me to learn and store the newly acquired information.  It is amazing to know how simple tasks that we as humans do without much deliberation is carried out by such intricate networks within our brains.  Learning and memory differs across different situations.  Learning to play an instrument activates different areas of the brain compared to learning calculus.  How do we know how to differentiate learning and how can we optimize our learning capabilities?  In addition, hopefully with further studies focusing on the neural networks behind learning and memory, those with learning disabilities such as ADD, Autism, or dyslexia will be able to learn more effectively and easily.  Just as how learning never ends, research on this topic will only further advance the field of neuroscience.   
    

  A video briefly explaining the neural mechanisms behind learning.